Vertical Takeoff and Landing (vertical lift) aircraft have long been considered desirable because of their ability to hover in flight and transition in and out of flight without a runway, in addition to flying in a horizontal direction. Although rotating-wing vertical lift aircraft (helicopters) have long been available, a rotating wing requires substantial clearance and can present safety hazards. In addition, rotating-wing aircraft generally have poor cruise performance compared to fixed-wing aircraft. Consequently, other types of aircraft, for example lifting off in a xe2x80x9ctail sittingxe2x80x9d configuration or employing xe2x80x9cfan in wingxe2x80x9d structure, are considered preferable in many situations.
During stationary flight, a rotating-wing vertical lift aircraft is supported by lift from air flow across its wing. Because the lift is developed across a relatively wide area, rotating-wing vertical lift aircraft possess some inherent stability against roll. Vertical lift aircraft other than rotating-wing aircraft do not enjoy such lateral stability because they are supported by a relatively compact source of thrust. For example, a ducted-fan type of vertical lift aircraft may be viewed, in operation, as sitting on a column of air. Although multiple thrusters can be employed for additional lateral stability, such an arrangement adds complexity and presents similar size disadvantages to those of a rotating-wing vertical lift aircraft.
Also, certain conventional types of fixed-wing vertical lift aircraft are capable of transitioning between vertical flight and horizontal flight while part or all of the vehicle transitions between a vertical and horizontal orientation with respect to the ground. Conventional approaches are problematic, however, when it comes to accommodating a payload while the vehicle makes the transition. Conventional methods for orienting the payload with respect to the rotating vehicle generally can be grouped into two broad categories: fixed payload and mechanically rotated payload.
Permitting the payload to rotate with a vertical lift aircraft from a horizontal to vertical orientation is generally undesirable. If the payload performs ground observation such as monitoring ground-based targets or tracking a vertical landing site, for example, compensations must be made while transitioning between horizontal and vertical orientations. If the payload includes humans, they must deal with the discomfort of moving between sitting and lying positions. These deficiencies have a compound effect when a pilot attempts to land vertically or visually track ground-based targets because of the combined disorientation and discomfort they cause.
Some conventional vertical lift aircraft have been developed in which all or part of the vehicle mechanically rotates with respect to the payload, thus permitting the payload to remain in a substantially fixed orientation while the rest of the aircraft rotates. When viewed from the perspective of the rotating portion of the aircraft, it is the payload that is mechanically rotated. A broad class of such vertical lift aircraft configurations can be categorized as having a mechanically rotated payload, including tilt rotor, tilt duct, and tilt wing. This class of aircraft presents a serious control problem, in that the mechanical rotations tend to be destabilizing and must be carefully coordinated with aerodynamic controls to keep the vehicle airborne. Furthermore, the mechanism needed to affect the rotation tends to be heavy, which adds weight and reduces aircraft performance. In addition, the mechanism represents a single failure point with potentially destabilizing failure modes. When considered together with the above problems, the typical complexity of the mechanism can present a serious safety hazard.
In view of the many problems associated with conventional rotating-wing and fixed-wing vertical lift aircraft, it would be desirable to have a new type of vertical lift aircraft.
A vertical lift flying craft according to various aspects of the present invention includes a lift unit that, during operation, develops a force including an upward component. A payload unit suspends from the lift unit. The payload unit (which may be an integral part of the vertical lift flying craft or provided as a removable object) suspends from the lift unit in such a way as to impart lateral stability while remaining capable of horizontal flight, without incurring the adverse effects of a downward pitching moment. In addition to a lift unit and a payload unit, such a vertical lift flying craft includes a pair of bearings and a suspension structure, which cooperate to suspend the payload unit from the lift unit. The bearings include two bearing members that are each rotatable with respect to each other about a rotational axis. The suspension structure includes two ends. One end of the suspension structure couples to one of the bearing members, while the other end couples to the payload unit.
By suspending the payload unit from the lift unit through bearings, the suspension structure permits the payload unit to move about the rotational axis, independent of the direction of the force developed by the lift unit. By permitting the payload unit to move in such a manner, for example to reach an equilibrium position when acted upon by wind resistance during horizontal flight, the suspension structure suspends the payload unit while avoiding the development of a downward pitching moment.
The bearing members are rotatable about a rotational axis, rather than as a universal joint. The payload unit is thus constrained from significant movement parallel to the rotational axis, which is perpendicular to the force developed by the lift unit. Consequently, a rigid moment arm is developed, about the roll axis of the lift unit, such that lateral stability is imparted to the lift unit. Advantageously, the lift unit does not need to be structured to have inherent lateral stability.
The lift unit is coupled to one of the bearing members so as to be capable of free rotation through an angular range (which may be limited, for example to 90 degrees) about the rotational axis. The lift unit includes a suitably configured thrusting system, an aerodynamic lift system, or both for developing a force to the lift unit in the direction of a force vector. By rotating through an angular range, the lift unit is capable of developing force as appropriate in various embodiments of the invention and in various modes of flight. For example, force may be applied in the direction of a vertical force vector (for hovering and vertical takeoff); in the direction of a horizontal force vector (for airfoil-assisted horizontal flight); and in the direction of a vertically angled force vector (for horizontal flight supported by the lift unit).
According to another advantageous aspect of the present invention, the suspension structure is coupled to the payload unit, at one end, through a bearing. By permitting the payload unit to rotate independent of the orientation of the suspension structure, such a coupling arrangement provides particular advantages. For example, when the end of the suspension structure is coupled to the payload unit (through the bearing) above the center of mass of the payload unit, the payload unit can be expected to maintain a constant orientation, regardless of the orientation of the suspension structure. Consequently, the payload unit may remain substantially horizontal even when pushed aft of the lift unit by wind resistance. Advantages of having the payload unit maintain horizontal orientation include reduced wind resistance and, in flying craft where the payload unit is dimensioned and configured to accommodate passengers, increased passenger comfort.